1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device, with which a low-resistance, heat-resistant, and highly integrated titanium polycide gate electrode having a gate oxide film with a high breakdown voltage can be obtained.
2. Description of the Prior Art
In an LSI, increases in yield and in number of chips per wafer can be expected by reducing the chip size, so the manufacturing cost is decreased. An increase in the degree of integration has been achieved by reducing the design size and increasing the chip size. In DRAMs of 256 Mbytes or more having a chip size exceeding 200 mm.sup.2, it is significant to suppress this increase in chip size.
Attention has been paid on the fact that when the resistance of word lines is decreased, the number of cells per word driver can be increased, and the number of word drivers can accordingly be decreased. For example, assuming that the resistance is 1/2 and that the number of cells per word driver is increased from 500 to 1,000 in a 1-Gbyte DRAM, the number of word drivers can be decreased by 500,000, so that the chip size can be reduced by about 5%.
Conventionally, in a DRAM, tungsten polycide obtained by stacking tungsten silicide on polysilicon has been widely used. As tungsten polycide has a high resistance, use of titanium polycide having a lower resistance, which is obtained by stacking titanium silicide on polysilicon, is sought for. When titanium polycide is applied to a DRAM, since the manufacturing process of the DRAM includes the step of processing the DRAM at a high temperature, a p- or n-type impurity is diffused from polysilicon to titanium silicide during the high-temperature process. This impurity diffusion degrades the heat resistance of titanium silicide. Then, the resistance increases, and the chip size may not be reduced.
Prevention of impurity diffusion to titanium silicide by intervening a titanium nitride film or the like between a polysilicon film and a titanium silicide film has been proposed, and studies have recently be made on a technique for a titanium polycide gate having a barrier metal.
In the titanium polycide gate, a titanium nitride (TiN) film barrier is formed between the titanium silicide (TiSi) film and the polysilicon (poly-Si) film usually by sputtering, so that impurity diffusion from the polysilicon film to the titanium silicide film is prevented, thereby preventing degradation in heat resistance during annealing.
If, however, the formed titanium nitride film has a low density, a sufficiently high isolation effect cannot be obtained, and the impurity, e.g., phosphorus, contained in polysilicon is sometimes diffused into titanium silicide during annealing. Generally, the section of a titanium nitride film formed by conventional sputtering has a columnar structure. In a low-density film, gaps are found among columns of the columnar structure (e.g., see "Thin Film Fabrication Handbook" issued by KYORITSU SHUPPAN CO., LTD.) It is assumed that the gaps among the columns formed in the titanium nitride film decrease the barrier performance of the titanium nitride film. In order to effectively prevent impurity diffusion from polysilicon to titanium silicide, the titanium nitride film must be imparted with a high barrier performance.
According to conventional ordinary sputtering, a titanium nitride film is formed under the following conditions. For example, the substrate temperature is 300.degree. C., the gas pressure is 3 mTorr, the ratio of Ar flow rate to N.sub.2 flow rate is 1:1, and the sputter power is 3 kW.
Meanwhile, it is known that, to increase the density of the titanium nitride film, thus improving the barrier performance, the titanium nitride film is preferably formed at a higher temperature.
Conventionally, the film density of the titanium nitride film formed by conventional ordinary sputtering changes depending on the substrate temperature during film formation. When the titanium nitride film is formed at a high substrate temperature, the density increases. Then, although the barrier performance improves, the breakdown voltage of the gate oxide film decreases.
FIG. 1 shows the relationship between the substrate temperature and the breakdown voltage of the gate oxide film. As shown in FIG. 1, when the titanium nitride film is formed at a substrate temperature of about 500.degree. C., the breakdown voltage of the gate oxide film decreases with a high probability. Thus, when the film formation temperature of the titanium nitride film increases, the breakdown voltage of the gate oxide film decreases.
More specifically, when the titanium nitride film is formed at a high temperature, it receives a large tensile stress, and a force having the same magnitude as the tensile stress but in an opposite direction to it is generated in the substrate due to the law of action and reaction. This force is assumed to act on the gate oxide film, thus decreasing the breakdown voltage of the gate oxide film.
Therefore, a method of forming a titanium nitride film, with which a stress generated during film formation is small so that the breakdown voltage of the gate oxide film will not decrease, is sought for.